One subject of the invention is novel materials of titanate type of perovskite or derived structure and their uses for the production of electrodes, more particularly in the cell elements of an SOFC cell (solid oxide fuel cell) or the cell elements of a high-temperature steam electrolysis cell (HTSEC), also known under the name of SOEC (solid oxide electrolysis cell). These novel materials have the distinctive feature of being able to be used as an SOFC cathode material, an SOEC anode material and also, after partial reduction, as an SOFC anode material or SOEC cathode material.
The SOFC cell elements constitute one of the most advanced systems for producing electricity with a high yield and without harming the environment. They may use hydrogen or a hydrocarbon such as methane as fuel.
Each cell element comprises two electrodes, an anode and a cathode, separated by an electrolyte.
Each electrode compartment must be composed of a material which satisfies several constraints: the microstructure must be stable during the production and use of the cell element; the various components of the cell element must be chemically compatible and have similar thermal expansion coefficients; the porosity and the catalytic activity must enable the cell element to exhibit good performances. More specifically, the anode and the cathode must be endowed with a high electronic conductivity.
Moreover, they must be stable under reducing conditions for the anode and under oxidizing conditions for the cathode. It is also desirable that the electrode material has an ionic conductivity.
The customary material for the production of the electrolyte is zirconium oxide stabilized with yttrium oxide (YSZ or yttria-stabilized zirconia).
The anodes of the SOFC cells are usually composed of a ceramic/metal (cermet) mixture. Nickel-based cermets, in particular cermets based on Ni and on yttria-stabilized zirconia (YSZ) have been developed and function remarkably with hydrogen as fuel. For cell elements functioning with hydrocarbons, Ni/cerine or Cu/cerine cermets have been developed more recently.
Ni/YSZ cermets nevertheless have many drawbacks: at high temperature they cause sintering of nickel particles, sulphur poisoning and deposition of carbon when the system functions with hydrocarbons. If, in order to prevent the deposition of carbon, a large amount of water is introduced at the top of the cell element, this leads to an accelerated growth of the nickel grains and eventually to a loss of the performances of the electrode.
In the case of the HTSEC cell elements, the use of Ni/YSZ cermet involves using a large amount of hydrogen in water used as fuel at the cathode inlet, in order to avoid oxidation of the nickel to NiO and/or Ni(OH) in particular, which would lead to a degradation of the electrode, especially during the shutdown phases of the system.
Several paths have been studied with a view to replacing the cermets in the cell elements of SOFC cells.
The titanate of strontium alone, only substituted by lanthanum (La) at site A, is not suitable as explained in Q. X. Fu et al., Journal of the Electrochemical Society, 153(4) D74-D83 (2006) and Olga A. Marina et al., Solid State Ionics, 149 (2002), 21-28.
Strontium titanates substituted by other transition metals appear to constitute a promising path, but their development requires improvements in the formulation of the material.
One of the solutions envisaged for limiting the premature ageing observed at the electrode/electrolyte interfaces is to develop novel materials which may be used both as cathode and as anode. This is because the introduction of one and the same electrode material means, above all, a single chemical and thermo-mechanical compatibility with the electrolyte to be controlled within the cell element. The symmetrical configuration furthermore allows a certain number of significant simplifications.
Specifically, the introduction of one and the same material for the electrodes should facilitate the reduction of the mechanical stresses within the cell element. It should also permit the simplification of the production method by the use of a co-sintering of the electrodes, which may limit the interdiffusion phenomena within the cell element, while making it possible to decrease the manufacturing cost and the sometimes tricky handling thereof. Thus cell elements that are more robust, more reliable and potentially less expensive should be able to be produced.
Another of the objectives that it is desired to achieve is the synthesis of a novel type of cermet containing a very small amount of nickel that has a purely catalytic role combined with a mixed (ionic/electronic) conductive ceramic matrix that moreover is basic, that is to say has a high “decoking” power in the presence of small amounts of H2O and/or CO2.
It has especially been sought to obtain a basic compound comprising a very small amount of catalyst, which is extremely divided and therefore extremely active, and distributed homogeneously at the porosity/ceramic interface.
In order to solve all of these problems it has been sought to develop a material that can be used for manufacturing the anode and the cathode so as to solve the problems of thermal, chemical and mechanical compatibility within the cell element and to simplify the manufacturing conditions of the cell element.
Symmetrical cell elements, using the same material as anode material and as cathode material, are known in the prior art: D. M. Bastidas, et al., Journal of Material Chemistry, 16, (2006), 1603-1605, highlighted the possibility of using a perovskite-type compound (La0.75Sr0.25)Cr0.5Mn0.5O3 (LSCM) as electrode material for an SOFC which can be used both as anode and as cathode in order to develop symmetrical cell elements. The first tests carried out on the symmetrical cell element (LSCM/YSZ/LSCM) show relatively good performances at 900° C.: 300 mW·cm−2 under H2 and 225 mW·cm−2 under CH4. In this precise case, it is the same material, stable under redox cycling, which is recommended as the anode and cathode material for an SOFC.
J. C. Ruiz-Moralez et al., Electrochimica Acta, 52, (2006), 278-284 have sought to optimize the micro-structure of LSCM-based electrodes in order to increase the performances of the LSCM/YSZ/LSCM cell element. They obtained, at 950° C., a power of 550 mW·cm−2 under H2 and of 350 mW·cm−2 under CH4. Even though they are already worthy of interest, these values are however not yet high enough given the operating temperature.
Another type of symmetrical cell elements La0.75Sr0.25Cr0.5X′0.5O3−δ/La0.9Sr0.1Ga0.9Mg0.2O2.85/La0.75Sr0.25Cr0.5X′0.5O3−δ (X′=Mn, Fe and Al) was developed recently by this team (J. Peña-Martínez, et al., Electrochimica Acta, 52, (2007), 2950-2958). They achieved a maximum power of 54 mW·cm−2 at 800° C. under (5%) humidified Ar/H2 for cell elements of the La0.75Sr0.25Cr0.5M′0.5O3−δ/La0.9Sr0.1Ga0.8Mg0.2O2.85/ La0.75Sr0.25Cr0.5Mn′0.5O3−δ type. These recent studies show the benefit of developing the technology of symmetrical cell elements.
Document WO 03/094268 describes doped strontium titanates and their use for producing electrodes of electrochemical cell elements and of devices such as SOFCs and SOECs.
Titanium may be substituted by Ni up to a maximum level of 20%.